In the field of the nuclear magnetic resonance imaging (MRI) apparatuses, those of the so-called open-type concept has being developed in recent years. These apparatuses can generally fall into the two types of the apparatuses: apparatuses using conventional cylindrical solenoid magnets with elongated axes for increasing the sense of openness, and apparatuses using open-type magnets for generating a uniform magnetic field between two opposite magnet assemblies to enable the access to the patient being imaged. The MRI apparatuses with the former magnets provide a greater sense of openness than the conventional apparatuses but the sense is still insufficient to meet the demand in the medical field.
The MRI apparatuses with the latter magnets can provide a remarkable sense of openness, which can make possible the so-called IVR (Interventional Radiology). International Laid-open Publication WO99/27851 “MAGNET APPARATUS AND MRI APPARATUS” discloses superconducting magnets suitable for such open-type MRI apparatuses.
The superconducting magnets consist of two magnet assemblies opposed across an MRI imaging area. Each magnet assembly has more than one superconducting magnets and is almost symmetric with respect to the z-axis. Main coils have alternately positive and negative pole coils and can generate a highly uniform magnetic field although they are compact.
The open-type magnets have, however, essentially lower efficiency of the magnetic field generation than conventional solenoid magnets and need a larger magnetomotive force of the magnets which may result in a high leakage magnetic field, as described in S. Kakugawa et al., “A Study on Optimal Coil Configurations in a Split-Type Superconducting MRI Magnet,” IEEE Trans. Appl. Supercond., Vol. 9, No. 2, pp. 366-369, 1999.
One of solutions to the above problem is the use of shield coils as described in the above patent, reference, and U.S. Pat. No. 5,883,558, entitled “OPEN SUPERCONDUCTIVE MAGNET HAVING SHIELDING.” Conventional solenoid magnets generally use the shield coils as lightweight and effective solutions.
Furthermore, as a method for shielding the leakage magnetic field, Japanese Laid-open patent Publication No. Hei 07-336023, entitled “SUPERCONDUCTING MAGNET DEVICE” discloses a superconducting magnet device consisting of a main coil for making a current in a constant direction flow, and a bucking coil for making a current in the reverse direction to the main coil flow, and correction coils for correcting the uniform degree of the magnetic field.
As described in the above patent and patent application, the shield coils can decrease the leakage magnetic field in the open-type magnets. However, more leakage magnetic fields may remain in the open-type magnets with the shield coils than in the conventional solenoid magnets with the shield coils, which may adversely affect the MRI apparatus installation in the hospital. More leakage magnetic fields may remain because the coils or permanent magnets for static magnetic fields may have a larger magnetomotive force, as described in the above reference, resulting in the larger magnetomotive force of the shield coils which may cause larger leakage magnetic field from the shield coil itself.
Therefore it is an object of the present invention to provide a magnet and shield coil assembly with a low leakage magnetic field.